XSLT: copy element text which is itself an xml doc without using value-of

Question

I'm copying the text content of an element I need (which is an embedded xml doc) and creating the new doc from the text, as shown below for a file format delivered to me that I don't control. The issue is that occasionally i get large (3MB+) text values (xml files) delivered in this one element and the parser crashes (java heap space) - I think its because the value-of cant handle the text as a string in one. I'd like to ideally just do a copy-of or some sort of identity transform just to strip other elements, or copy without buffering it into a string. Am I right thinking this is the issue, and is there a way? (without adding more memory).

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
  <xsl:output method="text" omit-xml-declaration="yes" />
      
  <xsl:template match="/">
    <xsl:value-of select="root/toplevel/row/payload" />
  </xsl:template>
</xsl:stylesheet>

<?xml version='1.0' ?>
<root>
    <toplevel>
        <row>
            <payload> 
                    &lt;?xml version="1.0" encoding="UTF-8"?>
                            &lt;documentProperties type="documentProperties">
                                &lt;producedBy>
                                    &lt;ourName type="string">NAMEHERE&lt;/ourName>
                                    &lt;user>Someone&lt;/user>
                                &lt;/producedBy>
                            &lt;/documentProperties>
            </payload>
            <System>NotWanted</System>
        </row>
    </toplevel>NotWantedEither
</root>

Note the text in the sibling and parent elements at the end is not wanted and does sometimes get included with several attempts at copying I've tried. I only want whats in payload. This code works with this example but not when the text exceeds some size limit.

Output :

                    <?xml version="1.0" encoding="UTF-8"?>
                            <documentProperties type="documentProperties">
                                <producedBy>
                                    <ourName type="string">NAMEHERE</ourName>
       ......
 <.... in practice +3 MB more content in output and source element text here...>
.......
                                    <user>Someone</user>
                                </producedBy>
                            </documentProperties>

Answer 1

Didn't find a desired XSLT solution and needed a working process fairly quickly. Adding more memory solved this for me. Increased heap space Xmx to 10G as a workaround for the odd time this happens.